Fiber optic collimating and focusing assemblies, sometimes known as collimators, are used to launch a beam of light from one optical fiber into free space, and then to capture such light and redirect it into another fiber. Collimators (i.e., optical devices for emitting parallel rays of light) are usually necessary whenever the free space propagation exceeds several hundred microns (“μm”; 1 μm=0.001 mm). For this reason, collimators are commonly used in fiber optic rotary joints (“FORJs”), such as shown and described in U.S. Pat. Nos. 6,301,405 B1, 7,142,747 B2 and 7,239,776 B2, the aggregate disclosures of each of which are hereby incorporated by reference. In a FORJ, the free space distance between the fiber optic collimating and focusing assemblies can be on the order of three to four inches [i.e., about 7.62 to about 10.16 cm].
In the past, collimator designs, such as shown and described in the aforesaid patents, have been selected for their simplicity, high pointing accuracy and reasonable optical performance. While such collimator designs have been sufficient for many conventional FORJs presently in use, there exists a desire to pass more and more data through existing FORJs.
Wavelength division multiplexing is a known technique for increasing the amount of data transmitted along an optical fiber. Basically, a plurality of input data signals are superimposed on a plurality of wavelength-separated optical carrier signals. The various optical signals are multiplexed, and are provided to the input end of an optical fiber. The multiplexed signals are transmitted along the fiber, and are demultiplexed at the exit end of the fiber back into the various component data signals. Wavelength division multiplexing is attractive because it allows a large amount of data to be transmitted along a single fiber by utilizing the bandwidth capability of the fiber. There are a number of known techniques. These include, but are not limited to: (1) conventional wavelength division multiplexing (“WDM”), (2) dense wavelength division multiplexing (“DWDM”), and (3) coarse wavelength division multiplexing (“CWDM”). Conventional WDM systems typically provide for up to sixteen channels in the third transmission window (C-band) of silica fibers at various wavelengths around 1550 nanometers (“nm”). DWDM systems typically use the same transmission window, but with denser channel spacing. CWDM systems, in contrast with conventional WDM and DWDM systems, use increased channel spacing to allow less-sophisticated and less-expensive optical multiplexer and transceiver designs. Thus, conventional WDM, DWDM and CWDM systems are based on the concept of using multiple wavelengths of light on a single fiber, but differ in the frequency of the wavelengths, the number of channels, and the ability to amplify the multiplexed signals in optical space. As used herein, the expression “wavelength division multiplexing” includes conventional WDM, DWDM, CWDM and similar techniques.
It is known to utilize wavelength division multiplexing to transmit data across fiber optic rotary joints. See, e.g., U.S. Pat. Nos. 5,991,478, 6,385,367 B1, 6,453,088 B1, 6,980,714 B2 and International Pat. Application No. PCT/US2006/016377 (published as Int. Pub. No. WO 2007/130016 A1), the aggregate disclosures of which are hereby incorporated by reference.
However, problems develop when high levels of optical power are transmitted through epoxy, silicone gel and/or index-matching media. For example, such epoxy and/or media will have a variable coefficient of thermal expansion, or a temperature-dependent refractive index that may adversely affect collimator performance. Moreover, exposure to high optical power densities can irreversibly darken the epoxy and/or the index-matching material. These detrimental effects have been observed at nominal power levels of about 256 milliwatts (“mW”). This represents a nominal energy density of about 3.41 gigawatts per square meter (“GW/m2”), based on a 10 μm diameter singlemode fiber. Since about ninety percent of the energy in a single-mode fiber is carried in a mode field having a diameter of about 5.8 μm, the resultant energy density in this reduced-diameter field is about 8.6 GW/m2. Testing has demonstrated that the maximum optical power capability of conventional collimators is approximately +15 dBm (i.e., about 30 mW). The typical requirements for WDM systems are well above this level.
In addition to volume effects (i.e., changes occurring within the optical path of the collimator), detrimental effects may occur on the surface of the singlemode fiber. This is particularly the case because the index-matching materials that would normally be in the interface between the singlemode fiber and the collimating ball lens, cannot be used. Because of the high energy density that exists on the surface of a singlemode fiber when a high power level is transmitted, contaminants (e.g., ceramic particles from ferrules and alignment sleeves, dust, and other contaminants) can be heated to temperatures higher than the melting point of silica, which may result in cracking and pitting of the fiber surface.
The high power transmission issue has been solved with the development of fusion-joint collimators where an end face of the singlemode optical fiber is fused directly to an abutting facing end face of a graded-index multimode lens, rather than being adhesively secured thereto, as by the use of an optical epoxy. However, there are inherent pointing accuracy issues with this type of design that precludes their use in a FORJ, where pointing accuracy is important.
Accordingly, it would be highly desirable to provide improved collimating lens assemblies that are suitable for use in FORJs and other applications, that can handle high power requirements, such as on the order of magnitude typically used for WDM, while still having a high degree of pointing accuracy, particularly when the optical signal will have to be propagated over several inches of free space from one fiber to another.